Sarcoplasmic MxA expression: A valuable marker of dermatomyositis

OBJECTIVE:To evaluate the diagnostic value of myxovirus resistance A (MxA) expression in the cytoplasm of myofibers in the diagnosis of dermatomyositis (DM). METHODS:We assessed the sensitivity and specificity of the sarcoplasmic expression of MxA in muscles with DM by immunohistochemistry in consecutive cases of DM (n = 34) and other idiopathic inflammatory myopathies (n = 1208 with polymyositis, 16 with anti–tRNA-synthetase antibody–associated myositis, 46 with immune-mediated necrotizing myopathy, and 50 with inclusion body myositis) and compared them with conventional pathologic hallmarks of DM, including perifascicular atrophy (PFA) and membrane attack complex (MAC) deposition on endomysial capillaries. RESULTS:The sensitivity and specificity of sarcoplasmic MxA expression were 71% and 98%, respectively. While the specificity was almost comparable to that of PFA and capillary MAC deposition, the sensitivity was higher, with PFA showing 47% sensitivity and 98% specificity and capillary MAC deposition showing 35% sensitivity and 93% specificity. Of note, in patients with DM with typical skin rash but no PFA, 44% of the samples showed sarcoplasmic MxA expression, which was higher than the 17% sensitivity of capillary MAC deposition in the population. CONCLUSIONS:Sarcoplasmic MxA expression detected by immunohistochemistry is a more sensitive marker of DM than the conventional hallmarks, indicating its practical utility in the diagnosis of DM. It may well be included in the routine immunohistochemistry panel for myositis. CLASSIFICATION OF EVIDENCE:This study provides Class II evidence that immunohistochemistry-detected sarcoplasmic MxA expression accurately identifies patients with dermatomyositis.